1. An alert or watchful person; a guardian who protects others: The first child of Mark and Nadine, now three years old, needed an argus as a babysitter who would be attentive and perceptive and keep little Tommy out of trouble when his parents were at work.
2. In Greek Mythology, a giant with 100 eyes who was the guardian of Io and was later slain by Hermes: When Jill was reading about Argus, she found out that after Argus died, Hera transplanted the monster's eyes into a peacock's tail.